Many of the chemicals used to impart flame resistance to textile materials, especially to thermoplastic textile substrates, are not water soluble and thus are usually applied by padding as aqueous dispersions or emulsions. Aqueous dispersions of water-insoluble, non-phosphorus-containing brominated aromatic or cycloaliphatic organic compounds and a metal oxide together with a latex or other binder are described in U.S. Pat. No. 4,600,606. These dispersions or emulsions require high levels of dispersing agents, surfactants, and sometimes organic solvents, in order to function effectively. Even so, dispersion or emulsion stability is often very concentration dependent and sensitive to the presence of other additives in the application bath. Also, the dispersing agents, surfactants, and especially the organic solvents can cause other difficulties in the treatment process, for example color loss of a dyed substrate being finished.
There are many methods for applying flame retardants to textile fabrics. The application method used depends on the characteristics of the flame retardant being applied as well as on its interaction with the substrate. For example, flame retardants that are water soluble cannot be applied by exhaustion from aqueous baths. Also, water-soluble flame retardants which have low boiling points cannot be applied by pad/dry/cure techniques due to the high loss of material during the drying step.
Powder coating techniques have been used in the past 20 years to apply a coating powder, usually a thermoplastic, more typically a thermosetting resin, onto a solid surface such as metal objects. Fluidized-bed coating and electrostatic powder-spray coating are but two illustrations. Powder-coating processes are fusion coating processes which require the powder particles to be fused or melted at some point in the coating process. The substrate to which they are applied must be capable of withstanding the temperatures needed to fuse or melt the coating powder particles, at least for momentary periods of time and in specific, limited, usually surface areas.
Coating powders and powder-coating processes offer a number of significant advantages: they are essentially 100% nonvolatile and no solvents or other undesired substances are given off during application and curing. The powders are ready to use and require no thinning or dilution with the attendant need for organic solvents, nor do they require complex emulsion or dispersion formulation. Coating thickness, hence flame resistance, can be easily controlled and the powder is well utilized. Overspray can be collected or filtered from the surrounding atmosphere and reapplied, an important consideration when the material applied is costly.
We have developed and hereby describe procedures for applying flame retardant chemicals in powder form onto textile substrates using powder coating techniques.